In several communications systems the data to be transmitted is compressed so that the available bandwidth is used more efficiently. For example, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172 and is incorporated herein by reference. The second, known as MPEG-2, refers to ISO/IEC standards 13818 and is incorporated herein by reference. A compressed digital video system is described in the Advanced Television Systems Committee (ATSC) digital television standard document A/53, and is incorporated herein by reference.
In information distribution applications, such as video on demand (VOD) and other applications in which a plurality of subscribers receive respective information stream(s), it is desirable to use variable bit rate (VBR) streams so that video image quality and link (network) utilization may be maximized. With VBR streams, the bit rate fluctuates with the complexity of the video. That is, more bits are given to complex frames and fewer bits are given to the easy to code frames. However, "serving" such VBR information streams provides a substantial challenge when implementing a cost effective video server. The primary difficulty is determining is what happens when the demand for bandwidth exceeds the channel capacity.
VBR MPEG is currently used in one or more satellite television systems. Such systems typically encode a set of channels (i.e., audio-visual programs) together, using a statistical multiplexer that drives the rate controller of each channel encoder. All the decisions about how to allocated the available bits are decided when the video is being encoded, just before transmission.
Unfortunately, the above-described encoding-level approach to bandwidth management does not work within the context of, e.g., a video server that typically delivers pre-encoded content, and typically has no control over which streams need to be sent over a given link. Thus, such video servers typically serve VBR streams by reserving disk and link bandwidth based on a maximum bit rate that the served stream will require. While this technique works well, the additional video quality is provided at the expense of underutilized link bandwidth. Moreover, in systems where multiple subscribers require information stored in a single mass storage unit (or array of mass storage devices), disk bandwidth (i.e., the maximum amount of data that may be retrieved in a given time period) may be exceeded by the aggregated bandwidth demands of the subscribers, especially where VBR streams are retrieved from the disk.
Therefore, it is seen to be desirable to provide a method and apparatus for providing high quality information streams while managing both link and disk bandwidth utilization within the context of a multiple subscriber or user information distribution system.